Coupling arrangement for high frequency amplifiers



arch 1935- R. H. VAN MINNEN ET AL 2,034,84

COUPLING ARRANGEMENT FOR HIGH FREQUENCY AMPLIFIERS Filed Dec. 18, 1933 3 Sheets-Sheet 1 Y INVENTORS P.H.VAN MINNEN P. A NORDLOHNE ATTORNEY 9 R. H. VAN MINNEN ET AL ,034,848

March 24- COUPLING ARRANGEMENT FOR HIGH PREQUENCY AMPLIFIERS Filed Dec. 18, 1933 3 Sheets-Sheet 2 INVENTORS R.H.VAN'P$INNENNE P-J. O DLOH ATTORNEY R. H. VAN MINNEN ET AL 2,034,848 COUPLING ARRANGEMENT FQR HIGH FREQUENCY AMPLIFIERS Filed Dec. 18, 1933 I5 Sheets-Sheet (5 M 5 R W Y ONL E ND N m m VNZ NA T M RP Y B Patented Mar. 24, 1936 y UNITED STATES PATENT OFFICE COUPLING ARRANGEMENT FOR HIGH FREQUENCY AMPLIFIERS Application December 18, 1933, Serial No. 702,852 In Germany December 21, 1932 8 Claims.

With high or radio frequency amplifiers it is imperative to avoid the reaction of the output circuit on the input circuit. For this purpose balancing condensers are commonly used, which are arranged in such a manner that they prevent the transmission of energy, which might occur through the grid-anode-capacity (sometimes also the filament-anode-capacity) of the tubes, by compensating for this inner tube capacity.

The use of such condensers has the drawback that the constructional difficulties and the price increase in accordance with the frequency and the voltage used, so that it is extremely difiicult and expensive to obtain the proper variable condensers for modern short wave power transmitters. Moreover, the comparatively long connecting leads of the balancing condensers interfere with the equilibrium of the arrangement.

The present invention has for its purpose to arrange either the high frequency amplifier or the frequency multiplication device in such a manner that the output circuit cannot react on the input circuit, without the use of such balancing condensers. This is achieved by providing that the reaction caused by the inter-electrode capacity (capacities) of the amplifying tube(s) is (are) balanced by the capacities between cathode and one or more of the electrodes thereof or of another tube.

A further object of the invention is to provide a vacuum tube amplifier arrangement especially adapted for the amplification of short radio frequency waves or impulses and in which an extra condenser, located either within or without the tube, may be used to increase the natural capacity between certain electrodes of the tube.

Other objects of the invention will become apparent to those skilled in the art from the following description taken in connection with the accompanying drawings, in which,

Figures 1, 4 and 6 represent schematically a balanced amplifier according to the invention;

Figures 1a, 4a and 6a represent the bridge arrangements realized in Figures 1, 4 and 6;

Figure 2 shows a frequency multiplication device according to the invention, Fig. 2a showing the corresponding bridge arrangement; 7

Figures 3 and represent single tube amplifiers according to the. invention; and,

Figures 3a and 5a represent the corresponding bridge arrangements.

With balanced amplifiers commonly a coup-ling arrangement is used, in which both anode-grid capacities of the tubes and two balancing condensers are inserted in the four branches, the output circuits being placed in both diagonals.

This principle is also used in the form of construction represented in Figure 1 in which, however, the usual neutrodyne condensersare re 5 placed by the anode-filament capacities of both tubes. For this purpose the filaments f1, f2 and the grids g1, 92 of the tubes are crosswise connected, whereas the input circuit is inserted between both connecting wires. Thus the grid- 10 anode capacities are compensated by the anodefilament capacities. For a perfect working .of this arrangement it is absolutely necessary that the cathodes of the tubes can acquire a high frequency potential. This is obtained by the radio frequency choke coils 3 which are inserted in the supply conductors of the filaments and constitute a large impedance to high frequency currents.

As shown in Fig. 1, the filaments are energized by the secondary winding 5 of a transformer the primary 4 of which may be supplied with alternating current from any desired source (not shown) As shown the mid tap of 5 is connected to ground by a lead 6. Two similar and equal may be connected to ground through condenser IT and adjustable tap connection 2!. The source of plate voltage is shown connected across opposite sides of a condenser 8, one side of which is shown grounded and the other side connected by an adjustable tap 22 to the midpoint of a coil In in the plate or output circuit. If desired coil I 0 may be tuned to the signal frequency by a condenser H. I I

If the sum of the anode-filament capacities of the tubes is equal to the sum of the anodegrid capacities, the circuit will be balanced and symmetrical as shown in Fig. 1a. If, as usually happens, the anode-grid capacity is the greater, a condenser l9 may be connected between the anode and filament to make up for the deficiency and efiect the balance.

From an inspection of Fig. 1a it appears that if an external source of potential is applied across the ends of coil ID, no current would flow through condenser I3 and therefore no voltage be produced across the ends of coil 14. As the opposite ends of this coil are connected to the grid and filament of each tube, it therefore follows that no voltage difference will exist between grid and filament or in other words, feedback between output and input circuits has been eliminated.

Also in the frequency multiplication device as shown in Figure 2 the filaments and the grids are crosswise connected as in Fig. l and the filaments can acquire a high frequency potential. In this case the direct plate current of both tubes passes through entire coil I 0.

In the arrangement represented in Figure 3 the invention is used in a single tube radio frequency amplifier. The input circuit is connected between the grid and the cathode, which cathode can acquire a high frequency potential. In this arrangement the anode-filament capacity is likewise used for compensating the anode-grid capacity. In this case a small condenser I9 is shown within the tube and out of the electron stream so that its capacity plus the anode-filament capacity will be equal to the anode-grid capacity.

It appears from the bridge arrangements represented in Figures 1a, 2a and 3a that the arrangements shown in Figures 1, 2 and 3 are not only free from reaction, i. e. that the output circuits cannot affect the input circuits, but also free from back-coupling, i. e. the voltages set up in the output circuit cannot affect the potential difference between grid and filament of the tubes.

In the arrangement represented in Fig. 4 the grid-filament capacities are used for compensating the grid-anode capacities. For this purpose the anodes and the filaments of both tubes are crosswise connected by leads 23 and 24. Also in this case the cathodes must be in a position to acquire a high frequency potential.

In the arrangement shown in Fig. 5 the anode-grid capacity is compensated by the gridfilament capacity for a single tube amplifier.

If according to Fig. 6 the opposite ends of coil IA of the input circuit be connected to the filaments, the anode-filament capacities must be compensated. This occurs through the gridfilament capacities by crosswise connecting the grids and anodes, through condensers 25, 26 as appears from this figure. Also in this case the cathodes can acquire a high frequency potential.

From the bridge arrangements represented in Figs. 4a, 5a and Get it appears that the corresponding arrangements are reaction free, it is true, but not free from back-coupling.

It will now be appreciated that a proper working of the arrangements referred to is only possible if the bridge is always balanced as accurately as possible. For this purpose the compensating capacity will consequently have to correspond to the compensated capacity. Experiments have proved, however that excellent results are also obtained with tubes in which this condition is not fully satisfied.

In Figs. 1, 2 and 3 it is necessary that af aa In Figs. 4 and 5 it is necessary that fu na In Fig. 6 it is necessary that fa af In order to satisfy these conditions as much as possible, tubes may be used in which one of the inter-electrode capacities is altered artificially thus obtaining, for instance, the relation afll: ua-

According to the invention such a capacity variation of the tubes is achieved by placing a small condenser within the tube whose armatures and supply conductors do not take part in the emission, as shown, for example, in Fig. 3. Since the dielectric is exhausted the condenser plates can be approached very closely toeach other and'have a small surface, while the supply conductors of the plates may be very small whereby the desired very small capacity may be secured.

It is also possible as shown in Fig. 1 to provide a small condenser outside the tube for compensating the capacity differences of the interelectrode-capacities of any of the tubes shown.

We claim:

1. In an amplifying arrangement, the combination of two vacuum tubes each having a cathode, grid and plate, a tunable input circuit including a coil, connections between the ends of said coil and said grids arranged to secure push-pull operation of said tubes, an output circuit connected between said plates and said cathodes, a heating circuit connected to the ends of said cathodes and including radio frequency choke coils and a connection between the grid of each tube and the cathode of the other whereby the capacity between the anodes and grids is neutralized by that between the anodes and cathodes.

2. The combination defined in the preceding claim in which the sum of the capacities between the anodes and filaments of the tubes is equal to the sum of the capacities between the anodes and grids.

3. In an amplifying device, the combination of a vacuum tube having a heated cathode and two cold electrodes, an input circuit tunable over a range of signal frequencies connected between said cathode and one cold electrode, an output circuit connected between the other cold electrode and said cathode, a connection external to said tube between said output and input circuits and a small condenser located within said tube envelope but outside of the electron stream and having connections within said tube between said cathode and one of said cold electrodes whereby the capacity between the two cold electrodes may be compensated.

4. In an amplifying arrangement, the combination of a vacuum tube having a cathode, grid and plate, a radio frequency choke coil connected to each end of said cathode, a source of heating current connected to the ends of said choke coils, an input circuit tunable over a range of frequencies connected between said cathode and grid and including a coil, an output circuit including a second coil connected to said plate, a circuit having capacity reactance only connecting a point on said second coil to the electrical center of the coil in said input circuit.

5. The device of the preceding claim in which a condenser has one side connected to said anode and its other side to said cathode, said condenser having a value substantially equal to the capacity between the anode and grid minus the capacity between the anode and filament.

6. In an amplifier the combination of two similar vacuum tubes each having a cathode, a control grid, and a plate, a pair of condensers connected in series across said cathode, a heating circuit connected to the ends of said cathode including means to permit said cathode potential to vary at radio frequency, a coil, a connection between one end of said coil and the common terminal of said pair of condensers, means coupling the other end of said coil to said grid, a coil connected to said plate and a source of high direct current potential connected to an intermediate point of said last named coil.

7. In an amplifying arrangement, the combination of two electron discharge tubes each having a cathode, grid and plate, a coil adapted to transmit radio frequency currents, means connecting the midpoint of said coil to ground, connections between the ends of said coil and said grids arranged to secure push-pull operation of said tubes, an output circuit connected between said plates and said cathodes, a circuit connected to heat said cathodes and including means to permit their potential to fluctuate at a radio frequency, and a circuit connecting the grid of each tube to the electrical center of the cathode of the other. 8. The combination defined in the preceding claim in which said output circuit includes a coil and a circuit having capacity reactance connecting intermediate points of said last and first named coils.

RINZE HENDRIK VAN MINNEN. PIERRE JANNE HENRI ALPHONSE NORDLOHNE. 

